1. Field of the Invention
The present invention relates to alarm systems which produce an audible signal and more particularly to electrical drivers used to operate such alarms.
2. Description of the Prior Art
The alarm system herein treated is designed for R8. in a smoke detector. A smoke detection cell and a circuit which monitors the condition of the smoke detection cell to produce an output voltage indicative of the smoke condition is described in the copending application of William Peil, Ser. No. 746,293, filed Dec. 1, 1976, entitled "Smoke Detector" and assigned to the Assignee of the present invention.
The foregoing monitor is designed for integrated circuit fabrication, using conventional bipolar transistor technology. The present arm driver is also designed for integrated circuit fabrication using conventional bipolar technology, and is suitable for integration on a common substrate with the monitor.
The present invention is for use in an alarm system which produces a loud sound to warn of a dangerous condition. It is designed for minimum energy consumption during operation or during stand-by. The most efficient sound producing mechanisms or alarms are ones which are designed for a limited range of frequencies. The frequencies should lie in the sonic range most audible to most individuals. Such an alarm may have a range of from 2,500 to 3000 hertz. In particular, wide range loud speakers are of low efficiency. The driver for such an alarm might take the form of an audio amplifier capable of faithfully reproducing an applied audio frequency signal. Such a design tends to be electrically inefficient, both in stand-by and under operating conditions. A second approach is to use a bridge type inverter to convert d.c. energy to a.c. energy at the desired frequency. While the conventional inverter is frequently designed for relatively high efficiency, it is normally not designed for the high reliability essential to alarm systems. Conventional driver circuits, unless certain measures are undertaken, stress the component transistors during switching and greatly limit their life expectancy. In addition, known inverters are rarely designed for the extremely low, microampere stand-by currents that are essential for reasonable life in battery operated systems.